Free paths and transport phenomena gases and the quantum theory of collisions. I.—The rigid sphere model

Abstract The necessity for the use of quantum mechanics in the theory of atomic phenomena is most clearly manifest in the study of collision processes. Diffrac­tion effects have been observed in the scattering of electrons from crystals and by atoms, while the recent developments of molecular ray technique have made it possible to establish the existence of cross-grating spectra in the reflection of molecular beams from crystal surfaces. In view of the importance of wave theory in these phenomena, it is clearly necessary to examine the conditions under which the classical theory of gases must be modified and to determine the nature of the modifications. Such an investigation receives added importance owing to the possibility of experimental test by molecular ray methods. Also, considerable interest is attached to the possibility of direct experimental proof of the Bose-Einstein statistics for neutral atoms and molecules from collision experiments as has already been possible for α-particles. In order to develop the quantum theory of collisions in a form suitable for this purpose, we first discuss the simplest model which bears sufficient re­semblance to the actual facts, and so we consider the rigid sphere model for gas atoms. This model has already proved valuable in the classical theory of transport phenomena and has the additional advantage of permitting an exact quantum mechanical solution. It will be seen that the results obtained by the use of this model are of great interest and suggest several new lines of investigation, both experimental and theoretical. Finally, a method for dealing with the general case of any law of force will be discussed.